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Introduction

1.1 Early Wind Measurements

Wind power has come a long way since the earliest windmills were built to grind corn for flour or wind pumps installed to drain the marshes for growing crops. The siting of these early machines was driven more by the practical necessity to be close to the local populace than for maximum economic benefit. Estimates of the local wind climate could mostly be obtained from local informed opinion or visual clues such as the angle of growth of trees, or indeed the complete absence of trees at extremely windy sites. Measurement of wind direction, as opposed to wind speed, has been made for far longer. One of the earliest-known examples of this is a weathervane in the shape of the god Triton which stood atop the Tower of the Winds in Athens when it was first built around 50 BCE or possibly earlier (see Figure 1.1). Weathervanes on buildings such as churches in the Western world have also been a common sight for centuries. Although giving an indication of the local wind direction at the time of observation, such early devices were not generally used in the recording of historical wind direction distribution. Before the development of instruments to measure the magnitude of the wind speed, human perception or slightly more objective visual indicators were used to provide a wind speed scale. For example, the famous English writer Daniel Defoe, following the Great Storm of 1703 which caused significant destruction in southern England, proposed an 11-point scale based on common phrases, as detailed in Table 1.1 (The Weather Window, n.d.). Scale point 6, describing 'a top sail gale', gives a clue to the importance of such information for the maritime community, and several such empirical wind speed scales devised by sailors were known to exist in the seventeenth century and probably much earlier. Possibly the most famous such scale was developed in 1805 by the Irish hydrographer Francis Beaufort, a Royal Navy officer who later became a rear admiral and trained Robert FitzRoy who, in turn, founded what later became the UK Meteorological Office. The so-called Beaufort scale was designed as a 13-point scale based on the impact the wind speed had on the sails of a ship. These initial visual descriptors would be later converted into an actual measurement scale, which is still in use today for some applications (e.g. shipping broadcasts) and is detailed in Table 1.2.

Figure 1.1 The Tower of Winds in Athens. Source: Joanbanjo / Wikipedia Commons / CC BY-SA 3.0.

Table 1.1 Daniel Defoe's proposed verbal wind speed scale from around 1704.

Table 1.2 The Beaufort scale with modern equivalent units of wind speed.

Smoke rises vertically

Sea like a mirror

Direction shown by smoke drift but not by wind vanes

Sea rippled

Wind felt on face; leaves rustle; wind vane moved by wind

Small wavelets on sea

Leaves and small twigs in constant motion; light flags extended

Large wavelets on sea

Raises dust and loose paper; small branches moved

Small waves, fairly frequent white horses

Small trees in leaf begin to sway; crested wavelets form on inland waters

Moderate waves, many white horses

Large branches in motion; whistling heard in telegraph wires; umbrellas used with difficulty

Large waves, extensive foam crests

Whole trees in motion; inconvenience felt when walking against the wind

Foam blown in streaks across the sea

Twigs break off trees; generally impedes progress

Wave crests begin to break into spindrift

Slight structural damage (chimney pots and slates removed)

Wave crests topple over, spray affects visibility

Seldom experienced inland; trees uprooted; considerable structural damage

Sea surface largely white

Very rarely experienced; accompanied by widespread damage

Medium-sized ships lost to view behind waves; sea covered in white foam; visibility seriously affected

Devastation

Air filled with foam and spray; very poor visibility

Source: taken from The Royal Meteorological Society (n.d.).

Table 1.2 describes three units of measurement including the knot (often abbreviated to 'kt', or 'kts' if plural) which may be less familiar. This is a unit of speed derived from the days of sailing ships when sailors would use a long length of rope to which pieces of wood would be tied using knots at regular intervals which would be paid out from the stern (rear) of a ship as it travelled for a defined period of time determined using an hourglass. The number of knotted pieces of wood paid out in this time period would be used to calculate the speed of the ship. The modern knot is defined as one nautical mile (1.852 km) per hour which is equivalent to 0.514 ms-1. This unit is still used to describe the speed of ships or aircraft and is sometimes used by meteorological agencies to measure wind speed, though the unit of ms-1 is more widely used nowadays.

These early wind measurements were primarily of interest for mariners where the wind was an important source of motive power. Today, it is generally only leisure shipping which still uses the wind to provide propulsion. However, now our attention has turned to use of the wind for generating electrical power and the quest to decarbonise the energy system. In this book, we look at the science of wind resource assessment from the point of view of measurement and modelling.

1.2 The Need for Wind Resource Assessment

The need for accurate assessment of the wind conditions at a site has been driven by the rapid expansion of wind power worldwide. By the end of 2020, a total capacity of 743 GW had been installed around the globe (Lee and Zhao, 2021). The global weighted-average installed cost for onshore wind energy in 2020 was $1355 kW-1 (IRENA, 2020). For a 100 MW wind farm this equates to a total investment of $140 million. The equivalent cost offshore is somewhat higher at around $3185 kW-1...